RE: When should I start using WCAG 2.0?

Hi,
 
Christophe's e-mail reminded me that there was something I didn't quite understand about the requirements for AAA. The Last Call Working Draft includes:

"WCAG 2.0 conformance at level Triple-A (AAA) means that all Level 1,
Level 2 and at least half (50%) of the Level 3 success criteria that
apply to the content types used are met assuming user agent support for
only the technologies in the specified baseline."
(http://www.w3.org/TR/WCAG20/complete.html#conformance-reqs)
 
My issues with the above are:

*	
	Why only 50% of the relevant Level 3 success criteria?
*	
	In achieving at least half (50%) of the relevant Level 3 success criteria, which of the relevant Level 3 success criteria should be met, and which can be ignored?

Regards

Dave Eadie